R1:adc2002d2870

Make it easier to parse "X-Forwarded-For" with one or more load balancers

Summary:
Fixes T13392. If you have 17 load balancers in sequence, Phabricator will receive requests with at least 17 "X-Forwarded-For" components in the header.

We want to select the 17th-from-last element, since prior elements are not trustworthy.

This currently isn't very easy/obvious, and you have to add a kind of sketchy piece of custom code to `preamble.php` to do any "X-Forwarded-For" parsing. Make handling this correctly…